1. Field of the Invention
The present invention relates to an agitator usable in batch processing of various viscous fluids performed in chemical, pharmaceutical and food industries to manufacture products in small quantities and in a variety of types. The agitator is also usable in processes in which, during the operation of the apparatus, reaction, dissolving or the like causes the liquid viscosity to change within a wide range, and the flow in the vessel to change from turbulent flow to laminar flow.
2. Description of the Related Art
The liquid flow characteristics within an agitator vessel greatly vary between a low-viscosity region (turbulent flow region) and a high-viscosity region (laminar flow region). Also, the manner of flowing and mixing varies between these regions.
In particular, in a low-viscosity region, the fluid and the blade rotate together. This phenomenon causes the formation of a solid-like rotary portion on the agitator axis, which may result in a mixing failure. For this reason, the provision of a baffle plate in the vessel is believed to be essential in general. The effect of the baffle plate, whose provision is essential for the low-viscosity region, diminishes, however, with increases in the liquid viscosity. In a high-viscosity region (laminar flow region), the provision of a baffle plate leads to the problem of a portion of the liquid remaining on and adhering to the back surface of the baffle plate. In the case of a low-concentration slurry liquid, a baffle plate is very effective to achieve uniform dispersion of solid particles. However, when the slurry concentration increases, the baffle plate acts to help solid particles to remain, deposit and solidify on the wall portion of the vessel interior.
Accordingly, when a process, etc. involving changes in viscosity within a wide range is to be performed, it has been the conventional practice to determine whether or not a baffle plate is to be provided in the agitator vessel for each of the low-viscosity, medium-viscosity and high-viscosity regions and to select a suitable shape of agitator blade accordingly.
Also, the conventional practice has coped with the above-described case by dividing the interior of the agitator vessel into a plurality of stages. When the shape of agitator blade is to be arranged for this purpose, a construction such as that shown in FIG. 6 has been adopted (Japanese Patent Unexamined Publication No. 57-45332). In this construction, an agitator vessel 1 has a helical ribbon blade 12 disposed therein, which is rotatable along the inner surface of the side wall of the vessel 1. Also, paddle blades 13 are radially provided on an agitation axis 2 at the center of the interior of the agitator vessel 1, which are rotatable in the opposite direction to that of the helical ribbon blade 12.
When the above-described case is to be coped with by using a baffle plate and arranging the shape of agitator blade, a construction such as that shown in FIG. 7 has been adopted (Japanese Patent Examined Publication No. 1-37173). In this construction, a flat-plate-shaped blade 6 is provided on a lower portion of an agitation axis 2 at the center of the interior of an agitator vessel 1, which is disposed along the inner surface of the bottom wall of the agitator vessel 1. A grating-shaped blade 7 continuing from the flat-plate-shaped blade 6 is provided on an upper portion of the agitation axis 2. Also, a plurality of baffle plates 14 are provided in a spaced relationship with each other on the inner surface of the side wall of the vessel 1, each baffle plate 14 extending axially from a lower position to an upper position of that inner surface.
With the agitator having the first construction where the helical ribbon blade 12 and the paddle blades 13 are used as the agitator blade, the agitator blade having a complicated structure makes operations such as charging, discharging and transferring difficult, thereby involving a risk of trouble
Another disadvantage is that since no effective blade is provided in the bottom portion of the agitator vessel interior, the liquid flow in the bottom portion is extremely inactive. Further, an agitating operation cannot be started by charging a small amount of liquid. Still further, since the paddle blades 13 disposed radially inward of the helical ribbon blade 12 are provided in a plurality of stages, the circulating flow generated by each paddle blade 13 in one of the stages collides with another circulating flow at the intermediate surface between these stages, thereby forming a remaining portion. Such a remaining portion acts as a boundary which deteriorates the degree of inter-stage mixing. Further, when the liquid level changes, this causes a change in the relationship between the position of the liquid surface and the position at which the blades are mounted. Thus, a change in the liquid level leads to a difference in the mixing condition. Furthermore, a flow discharged by a paddle blade 13 in the radial direction of the vessel hinders a downward flow caused by the radially outward helical ribbon blade 12. As a result, the overall circulating flow inevitably becomes insufficient.
The agitator having the second construction where the agitator blade consisting of the flat-plate-shaped blade 6 and the grating-shaped blade 7 continuous therewith is used together with the baffle plates 14, does not entail the disadvantages of the first agitator, and the apparatus is advantageous in that the flow generating characteristics of the agitator blade enable a reduction in the mixing period, and that the applicable viscosity range is wide. However, the provision of the baffle plates 14 inevitably leads to problems such as those described above, that is, formation of a remaining portion on the back surface of the baffle plates when the liquid viscosity increases, occurrence of flow failure in a high viscosity region, and remaining, deposition and solidification of solid particles on the wall portion of the vessel interior when the slurry concentration increases.